Bridgedeck centreboard why don't they work???

That would be a VERY difficult thing to work out. Want an example, go look up Tom Speer's postings on gybing boards.

 

Of course you could always employ my aftmast design that eliminates a BIG leeway inducer....the mainsail....ha...ha

 

I'm confused by those videos as well,...just not enough info as to the basic parameters,...and UpOnStands please tell me what that 'ruler?' is swapping thru the video??

 

As concerns the topic of ventilation of a surface piercing foil, that paper is of VERY little use. At the end it gives a little diagram as to how it might work but no explanation, or any details.

I find this video you posted much more interesting, and I have submitted a comment to its creator to answer a few question about it, and possible join our conversation here.
https://www.youtube.com/watch?v=zJttVDFbYJY

 

Surface-Piercing Hydrofoil Ventilation Inception

I find this paper rather interesting, although it again is very concerned with vessel lifting foils more then surface piercing ones. I'll excepet a few portions that might be applicable to reduce the ventilation of our boards.

CFD Analysis of Surface-Piercing Hydrofoil Ventilation Inception

 

Hi Brian,

Without entering into complicated talks about the boundary layer, pressure gradients etc., for practical purposes it is IMO sufficient to know following few facts about the ventilation:

1) The onset of surface-piercing foil ventilation can be triggered by any water-surface perturbation (waves, aerated water, wake of objects in front of the foil etc.), when the foil is working under a sufficiently high hydrodynamic load (in this case, lift). And, for a given speed, hydrodynamic load is proportional to the AoA.

2) The onset of ventilation is particularily sensitive to:

a) loading of the foil area closest to the water surface;
b) pressure peak immediately behind the leading edge of the foil.
​

3) Once the water surface is breached by the air, the air stream propagates downwards along the path connecting of minimum pressure, approximately until the point where the atmospheric air pressure equals the local static pressure on the foil (which can be quite deep).

Knowing these basics, one can draw some conclusions about ways to minimize the probability of ventilation.

First, minimize flow perturbations. In practice it means - avoid placing water-piercing objects in front of the centerboard. That's all you can do about it. The other forms of perturbations are out of your control (waves, air bubbles etc.).

Next, you can decrease the hydro load on the centerboard in the proximity of the water surface. In practice it means decreasing the camber (variable-section centerboard) and/or decreasing AoA (variable twist). From the practical (constructive) point of view, both are a demanding and hardly doable solutions, and I am mentioning them for the sake of completeness.

Third, you can decrease the pressure peak by adopting foil sections with the point of max. camber and thickness placed more towards the trailing edge. NACA laminar sections (6-series or 7-series) could be a good place to look, for example.

Fourth, increase the depth (span) of the centerboard. This increases the probability that deeper parts of the foil will remain washed by a clean flow even when the more superficial ones are ventilated. The drawback is an increased root-bending moment, which requires heavier, bulkier and more costly centerboard scantling.

Finally, impede the propagation of the air stream downwards along the span. It is done by placing one or more fences (plates) along the span of the foil.
A single fence is used when the water surface is at a known fixed height. This is not your case, since wave motion can be significant in between the two demi-hulls of the cat, so you actually don't know at what point along the span will the foil pierce the water surface at a given moment.
For that reason, a series of fences is IMO a better option. The first one should be placed some 10 cm below the still water line, and a couple of additional ones should be added down to the lowest expected water level along the centerboard span. Since the ventilation propagates along the suction side of the foil, it is sufficient to place them on the outer side of each of yor centerboards. In this way, there will be no mechanical interferences between centerboards.
As an example of multiple anti-ventilation fences, see the hydrofoil of Hydroptere:

This is IMO probably the most effective and simplest thing you can do to minimize the ventilation on your centerboard. In this CFD work you can visually appreciate the difference in ventilation levels between a clean foil and the one one with a series of fences: https://bib.irb.hr/datoteka/837663.Sorta2016-ventilation.pdf

Hope it helps.

Cheers

 

sensible stuff, so any lateral stays to the board would have to be well clear of the water but with wave action that seems very difficult to achieve and keep a reasonable level of brace resistance.

Daiquri, how is this for first pass analysis?
10m yacht with sail area of 35 sq m apparent wind of 10 m/s at 25deg apparent, the heeling force is 252 kgf (from SailPowerCalc) add another 100 kgf for superstructure drag so total foil lift needed is 352 kgf.

for a reasonable board profile but still built in carbon, this demands a foil area of 2.8 sq m. Boat speed is 4.8 knots.
So for 1 m chord length of foil is 2.8 m. To which we add a fudge factor to account for free surface effects, say 1 m?, plus the free space gap to the pivot of the board, say 70 cm, so load moment is 352x(1+0.7+2.8/2) = 1091 kgm. Plus the fall sideways event + safety factor = some really serious stuff to resist.

Edit. Shuttleworth's rule of thumb is in-hull dagger board area = 2% of sail area. So board area is more like .7 sq m? This significantly drops the loading. http://www.boatdesign.net/forums/mu...oard-vs-double-boards-37787-6.html#post461810

 

How about doing a variation on the motor sled idea? It conforms to the surface of the water and is braced for lateral load. This gives a better endplate than a nacel and still allows kickup/retraction. Somebody could get creative and work in a motor too I suppose, of course if things go horribly wrong there you lose your leeway prevention and engine.

 

just off the top of my head the surface drag loads and weight seem excessive.
Better to check if its possible to support the loads of the pivoting free span foil. If that proves to be impossible, then go to plan B.

 

Not me, I believe in through hull boards but thought it would be fun to contemplate. A longer thin pod perhaps once the length beam drag is calculated. At some point it becomes an articulated hull..

It is hard to come out ahead of an in hull board. If a centerboard is fenced then you lose the ability to rake the board fore and aft for trim etc....

 

Like Daiquiri I have been of the opinion that ventilation fences are the neatest solution to the ventilation issue with the between the hulls centreboard however it kills the possibility of canting the board forward or aft as the fences will no longer be parallel to the waterline, or does it ?

 

Cav, snap !

 

Mr Daiquiri;

Do you put no value in canting the board forward as has been mentioned to counter ventilation ?

 

There are ways..... but how heavy. Our articulated hull could adjust for and aft....

A fenced deck dagger in a bridge deck cassette could have the cassette mounted on rails/channel for for and aft trim. kick up like a rudder cassette in case you prong it.

A point I'd like to point out is in the PNW wilds we have floating debris fields at times and one of the piloting options is taking drift between the hulls.

 

Isn't it about 5 degrees on a transom rudder? That occurred to me too.